The present invention relates to improvements in spray guns, and in particular to an air spray gun for spraying two component chemically reactive materials.
One type of spray gun for plural component materials is disclosed in U.S. Pat. No. 3,366,337 to Brooks et al. In that spray gun, which is of the airless type, means are provided for intimately mixing and spraying two liquid reactants immediately prior to the situs of application. This accomplished by impinging jets or streams of the reactants against each other under pressure from opposed directions into the rearward end of a relatively large mixing chamber within the gun, whereby the reactants are mixed within the chamber and then discharged through an airless outlet orifice at a forward end of the chamber.
At the end of a spraying operation with the spray gun of said U.S. Pat. No. 3,366,337, mixed reactants within the chamber must be quickly removed before completion of the reaction and formation of a blockage in the gun. To that end, means are provided for introducing a stream of solvent into the rearward end of the chamber after the gun has been operated, so that the chamber and outlet orifice can be cleaned of residual material to enable further spraying operations. Disadvantages of the technique are that a separate container is usually required to collect the discharge during cleaning, it can happen that solvent contacts and mars a finished product, the use of solvent adds cost to the operation, and in view of environmental considerations, it is not desirable that solvent be sprayed into the air. In addition, cleaning of the chamber with solvent is often less than thorough, with the result that the gun must be disassembled to remove hardened material, and customary practice contemplates soaking the gun in solvent to remove mixed reactants whenever the gun is not going to be used for a period of time. Also, with some types of modern day material, mere impingement together of reactants in a chamber does not provide satisfactory mixing of the materials, and airless spraying of the materials requires the provision of airless pumps and often does not result in a degree of atomization that is satisfactory.